What is the motivation of a MMORPG?
-To get stronger in any way possible(with friends , get a stronger group ,to be stronger individual ,to have more friends, to get monetary power , to be able to create better thins, to own more thins, to defeat more dangerous foes, etc etc etc)
is a lv 50 stronger than a lv 15 ?
yes.
Is there a lot of thins to do at lv 50 ?
no.
Is it really all that difficult to understand ?The question itself on op is rather silly.

Ps; Kilta_Firelotus really stop preaching , you dont play the game, stop talking like you know everything.When you have put enough time in the game ill respect your high and mighty opinion and defense force of Se Gods.,until then a bit of modesty would not hurt you one bit.